In cosmogony, a protoplanet is a quasi-planetoid which is slightly larger than a planetesimal and orbits within a solar nebula's protoplanetary discs. Such objects are formed by the accretion of objects (such as dust and planetesimals) within the accretion disc of a solar nebula. Early protoplanets are generally relatively hot and liquid. As the protoplanets cool into planets, their heavier elements will sink to the center; while lighter elements rise to the surface; such a process is known as planetary differentiation.